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Normal incidence surface impedance
The normal incidence surface impedance is a complex coefficient given by the acoustical pressure to velocity ratio at the surface of the tested sample when it is excited by a normal incidence acoustical wave. It measures the resistance and inertia encountered by the acoustical wave trying to penetrate the material. Its real part is the acoustical resistance, and its imaginary part is the acoustical reactance. It is particularly useful when studying the effects of an air cavity backing a resistive layer.
Equipment: Impedance tube
Standard: ASTM E 1050, ISO10534–2
Method: Impedance tube and the transfer-function method [1,2]
Property: Z - surface impedance (Pa.s/m)
Tested frequency range
- Small impedance tube (29 mm diameter) : 500 to 6 000 Hz
- Medium impedance tube (44.5 mm diameter): 300 to 4 100 Hz
- Large impedance tube (100 mm diameter): 50 to 1 600 Hz.
Comment
The normalized surface impedance (Zn) is often used. It is defined by Zn=Z/Zo, where Zo is the specific impedance of air. Zo is given by Zo=ρc, where ρ and c are the density and sound speed in air.